Institute of Physics

Wall painting from the tomb of Djehutihotep. A large statue is being transported by sledge. A person standing on the front of the sledge wets the sand. (Source: Al-Ahram Weekly, 5-11 August 2004, issue 702)

Ancient Egyptians transported pyramid stones over wet sand

29 April 2014

Physicists from the University of Amsterdam have discovered that the ancient Egyptians used a clever trick to make it easier to transport heavy pyramid stones by sledge, allowing them to halve the number of workers needed. The researchers published this discovery in the prestigious journal Physical Review Letters.

For the construction of the pyramids, the ancient Egyptians had to transport
heavy blocks of stone and large statues across the desert. The Egyptians
therefore placed the heavy objects on a sledge that workers pulled over the
sand. Research from the University Amsterdam has now revealed that the Egyptians
probably made the desert sand in front of the sledge wet. Experiments have
demonstrated that the correct amount of dampness in the sand halves the pulling
force required.

Firm sand

The physicists placed a laboratory version of the Egyptian sledge in a tray
of sand. They determined both the required pulling force and the stiffness of
the sand as a function of the quantity of water in the sand. To determine the
stiffness they used a rheometer, which shows how much force is needed to deform
a certain volume of sand.

Experiments revealed that the required pulling force decreased proportional
to the stiffness of the sand. Capillary bridges arise when water is added to the
sand. These are small water droplets that bind the sand grains together. In the
presence of the correct quantity of water, wet desert sand is about twice as
stiff as dry sand. A sledge glides far more easily over firm desert sand simply
because the sand does not pile up in front of the sledge as it does in the case
of dry sand.

Wall painting

The Egyptians were probably aware of this handy trick. A wall painting in the
tomb of Djehutihotep clearly shows a person standing on the front of the pulled
sledge and pouring water over the sand just in front of it.

Besides revealing something about the Egyptians, the results are also
interesting for modern-day applications. We still do not fully understand the
behaviour of granular material like sand. Granular materials are, however, very
common. Other examples are asphalt, concrete and coal. The research results
could therefore be useful for examining how to optimise the transport and
processing of granular material, which at present accounts for about ten percent
of the worldwide energy consumption.